Retroreflective sheeting has particular use in making highway signs, street signs and the like, and is now employed extensively. The Federal government has recognized two primary types of retroreflective sheeting: glass bead and cube-corner. Such approved sheeting materials are found in a specification entitled "FP-79", published by the U.S. Department of Transportation, Federal Highway Administration. Specification FP-79 presently has been adopted as a purchasing standard by many state highway departments, and it sets forth certain minimum specifications which must be met by retroreflective sheeting of the cube-corner type. Included among the specified characteristics are those for reflectivity, color, flexibility of material and resistance to cracking and weathering.
Cube-corner type reflector elements generally provide a higher specific intensity at 0.2.degree. observation angle and 0.degree. entrance angle than do glass bead type reflector elements, but, to applicant's knowledge, no one successfully has furnished a sheeting material in commercial quantities which generally will meet the requirements for the Class IIB sheeting set forth in the aforementioned FP-79 specification. It therefore is a primary object of the present invention to provide a unique sheeting product which will substantially meet such specified criteria and which can be produced in accordance with the novel methods disclosed herein in an economical fashion and in commercial quantities.
Retroreflectivity is achieved by cube-corner type reflector elements primarily through the principle of total internal reflection. It is well known that any surface contact made by another material with the faces of the cube-corner elements generally has a deleterious effect on the reflectiveness of the reflector element.
However, when all of the element faces are metallized, or mirrored, then, rather than relying upon total internal reflection, retroreflection is achieved by specular reflection from the mirrored faces. Generally, metallizing will provide a grayish or black coloration under certain daylight onditions vis-a-vis unmetallized cube-corner type elements.
The present invention relates generally to methods and apparatus for producing retroreflective sheeting constructins and, more particularly, to methods and apparatus for producing a flexible laminate sheeting construction including an upper thermoplastic sheet, the reverse of which is provided with a repeating, retroreflecting pattern of fine or precise detail, a backcoating to protect the formed pattern, and a selectively applied intermediate layer allowing bonding of the backcoating to overlay the formed pattern on the thermoplastic sheet while preserving and enhancing the retroreflective properties of both the formed pattern and the laminated sheet. More precisely, the present invention is applicable to the production of cube-corner type retroreflective sheeting laminates.
Within the art of designing reflectors and retroreflective material, the terms "cube-corner" or "trihedral," or "tetrahedral" are recognized in the art as describing structure or patterns consisting of three mutually perpendicular faces, not limited to any particular size or shape of the faces, or the orientation of the optical axis of the cube-corner element. Each of the cube-corner faces can assume a different size and shape relative to the others, depending upon the angular reflective response characteristics desired, and the cube forming techniques employed.
Examples of prior cube-corner type reflectors may be found in U.S. Pat. No. 1,906,655, issued to Stimson, and U.S. Pat. No. 4,073,568, issued to Heasley. Stimson shows reflex light reflector including an obverse face and a reverse light-reflecting face consisting of a plurality of cube-corner type reflector elements with each such element having three mutually perpendicular surfaces adapted for total internal reflection of light impinging thereof from the obverse face. Heasley describes a cube-corner type reflector in the form of a rectangular parallelpiped.
It long has been desired to obtain the benefits of cube-corner reflective properties in the form of fexible sheeting. As noted above, one advantageous aspect of such sheeting is in the manufacture of highway and street signs, markers and the like, where graphics are printed, painted, silk-screened or otherwise applied to a highly reflective substrate mounted to a flat, stiff, supportive surface. Flexible retroreflective sheeting, when used as such a substrate, can be stored and shipped while wound onto rolls, and can readily be cut or otherwise formed into the desired shape and size required for a particular application. The reflective nature of the sheeting allows such signs, markers, and the like to reflect light from a vehicle's headlights, permitting the item to be read by the driver, without requiring a permanent light source to illuminate the sign or marker.
Production of such retroreflective sheeting has been made practicable by apparatus and methods to form precise cube-corner patterns in greatly reduced sizes on flexible thermoplastic sheeting. Desireably, such sheeting may then be assembled in the form of self-adhesive laminates.
Others have recognized the desireability of producing retroreflective thermoplastic material in sheet form. U.S. Pat. Nos. 2,310,790, 2,380,447, and 2,481,757, granted to Jungersen, described and teach the shortcomings of previously-known reflectors manufactured from glass, and the advantages inherent in providing a reflective material in a less fragile and more flexible sheet form. While so suggesting, it is not known if Jungersen in fact ever commercialized any product disclosed in such patents.
In U.S. Pat. Nos. 4,244,683 and 4,332,847 issued to Rowland, the desirability of manufacturing cube-corner retroreflective sheeting in a continuous, nonstop process is presented, but the approach selected by Rowland is a "semi-continuous" process (Rowland U.S. Pat. No. 4,244,683, column 2, lines 18-38), presumably so-called because the process requires frequent repositioning of the molding plates.
In U.S. Pat. No. 3,187,068, issued to DeVries, et al., continuous production of reflective sheeting is disclosed, utilizing encapsulated glass microspheres as the reflecting medium. DeVries, et al. describes the application of a pressure-activated adhesive layer to such sheeting to enable attachment of sheeting setments to selected surfaces.
In U.S. Pat. No. 3,649,352, issued to Courneya, a beaded sheeting construction is described, portions of which become reflective when heate, and which includes a pressure-activated adhesive layer allowing attachment of the sheeting construction to other articles.
Palmquist, et al. U.S. Pat. No. 2,407,680 teach the utilization of glass microspheres of beads included as the reflective elements in flexible sheet forms; Tung, et al., in U.S. Pat. No. 4,367,920, also describes a laminated sheet construction using glass microspheres as the reflective elements.
A common problem in the construction of reflective laminate sheeting is to find means to bond the layers firmly together in a way which preserves the required retroreflective qualities of the reflective elements selected for use. An example of prior effects to solve this problem with respect to glass microspheres may be seen in U.S. Pat. No. 3,190,178, issued to McKenzie, wherein a cover sheet or film is secured over exposed glass microspheres by use of die elements which force a portion of the material in which the glass microspheres are embedded into contact with the cover sheet. The die elements thus create a grid pattern on the resulting sheeting construction, with each grid forming a separate cell. Within each cell, an air space is maintained between the microspheres and the cover sheet, and incident light traverses the cover sheet and the air space to be retroreflected by the embedded microspheres.
Holmen, et al., U.S. Pat. No. 3,924,929, teach a cube-corner type upper rigid sheet having upstanding walls, or septa, integrally formed as part of the cube pattern. The septa extend to form a regular geometric pattern of individual cells, with the septa extending at least as far from the upper sheet as the cube-corner elements. A particulate packing may be used to fill each of the cells, and a backing sheet is then attached to the rear of the upper sheet, with the septa service as the attachment sites. Holmen, et al. use relatively large cube-corner elements fashioned as rigid sections bound to a flexible back, and has limited flexibility in use.
In McGrath, U.S. Pat. No. 4,025,159, the cellular concept is described with respect to cube-corner type retroreflective sheeting, through us of dies to force a carrier film into contact with the reverse side of the cube-corner sheeting. The carrier film must then be cured with radiation to bind it to the cube-corner sheeting and, as in McKenzie, the resulting cells include an airspace extending between the carrier film and the reverse side of the cube-corner sheet. The air cell structure apparently was intended to provide a hermetically sealed cell, avoiding the need for metalizing the cube-corner elements, and providing an air/thermoplastic interface to enhance retroreflection.
None of the foregoing teach the assembly of molded or embossed cube-corner type retroreflective sheeting into self-adhesive laminates which protect and enhance the reflective properties of the sheeting without requiring the use of dies or of integrally-molded septa or walls included as part of the cube pattern. Further, none of the foregoing permits the material to benefit from encapsulated sections of cube-corner elements while enhancing and substantially meeting the requirements specified in the aforementioned DOP FP-79 Specification.